A fluorescent screen for a cathode ray tube for televisions is usually prepared by a series of operations which comprises forming by a photo printing method three-color emitting phosphor picture elements R (red), G (green) and B (blue) on the inner surface of a face plate provided with a black matrix, subjecting the surface of the phosphor picture elements R, G and B thus formed to filming treatment, then vapor depositing aluminum to provide a metal back to increase the light-taking out effects of the fluorescent screen, and finally burning a filming material and an organic substance for photo curing such as polyvinyl alcohol remaining after the photo printing in the picture elements, by baking treatment.
However, in such a series of operations, if copper is included even in a very small amount at a part of the blue picture element, such a part undergoes a color change from the initial blue-emission to a green-emission, and green specks will be formed in the blue picture element, thus leading to a coloring defect of the blue picture element.
This color change of the blue picture element to green is believed to be caused in such a manner that the copper or its compound included in a very small amount undergoes ion diffusion due to the heat during the baking treatment, whereby the blue-emitting phosphor ZnS:Ag undergoes a compositional change to a green-emitting phosphor of ZnS:Cu or ZnS:Cu,Ag, which will form green specks.
Under the circumstances, causes for this copper contamination have been examined. It has been found that there are an external factor such that copper powder or a copper compound is included from outside during the process of forming the fluorescent screen and an internal factor such that, for example, a copper compound is contained in the phosphor itself during its recovery process. The color change of the fluorescent screen by such a copper contamination source is considered to be due to a chemical change during the filming treatment or during the mixing of a photosensitive slurry of the phosphor and due to the copper ion diffusion by heat during the baking treatment.
A method for preventing copper contamination by controlling the diffusion of copper ions during the filming treatment, is disclosed in Japanese Unexamined Patent Publication No. 243884/1986.
However, there has been no effective measure to prevent copper contamination due to the copper ion diffusion by heat during the baking treatment.